In the manufacture of sheet materials, at speeds up to 3000 feet per minute in the case of paper and typically 500 to 1000 feet per minute in many plastics manufacturing procedures, the thickness of the sheet material being produced is of critical importance. In the past, many different approaches have been used to monitor sheet thickness.
One such approach has been to provide scanning devices that repeatedly traverses back and forth across the rapidly moving web of sheet material being produced. Such scanning devices may include a radiation source (e.g., krypton or strontium) mounted below the web and a detector (e.g., an ion chamber) mounted above the web to measure the radiation that passes from the source and upwardly through the web. Such devices are sensitive to the total mass between the source and detector; and the detector output thus is dependent not only on the thickness (and composition) of the web but also on air in the various gaps. In many applications, particularly when thin webs are involved, the effect of the air on the total radiation passing from the source to the detector may be as great, or greater, than that of the web whose thickness is to be monitored. Further, the effect of the air is highly dependent on air temperature, and the temperature of the air in the gaps on either side of the web may change rapidly.
Typical prior efforts to account for changes in the air temperature have involved the use of thermistors to monitor the air temperature in the various gaps. These efforts have not been entirely satisfactory, particularly for the air gaps on either side of the web. In these gaps the air temperature may rapidly change, but the time constant of the thermistors is typically about 10 seconds. Further, the thermistors are usually mounted in such a way that, unless some additional mechanism is introduced to insure air flow, the air in the region surrounding the thermistors may circulate poorly.